The present invention relates to a method of controlling the movement of a support relative to a base, comprising a central control unit for generating target-condition values in the coordinates of an orthogonal coordinate system of said base for moving the support and further comprising movement devices for moving the support, the degrees of freedom of the movement devices defining a non-orthogonal coordinate system. In addition, the present invention relates to a device for controlling the movement of a support relative to a base as well as a machine tool provided with a machining device.
Such methods and devices are particularly suitable for machine tools with non-orthogonal axes of movement, the workpiece being secured in position on the base and a machining device being provided on the support. The relative movements required for producing the workpiece are normally carried out by moving the support relative to the stationary base, but it is also possible to carry out a feed motion with the base in one or in all directions of movement. Such devices and methods are advantageously used for machine tools having non-orthogonal feed axes and a closed-loop kinematic structural design. These devices and methods are particularly suitable for use in hexapodal machining centers in which the support is articulated on the base via six struts of adjustable length. The movement of the support in all spatial degrees of freedom is effected by a controlled adjustment of the length of the struts, a non-orthogonal coordinate system being defined by the changes in length along the six linear axes. In addition, such devices and methods are suitable for use with manipulation devices for positioning and transporting workpieces. The longitudinally adjustable struts can also be replaced by struts having a fixed length; in this case, e.g., the foot points of said struts are displaced. Also, a combination with axes of rotation is possible.
In the case of movement devices, the movements to be carried out are normally related to a fixed orthogonal coordinate system corresponding to the degrees of freedom of movement of the individual movement devices. For producing a work-piece in a machine tool, this coordinate system is zeroed to a reference point of the workpiece before the machining operation begins. This permits a simple programming of the movement to be carried out and a comparatively small computing effort when the program is being executed in the machine.
In the course of more recent developments in the field of machine tools, machines with non-orthogonal feed axes have lately been suggested; in comparison with conventional machine tools with orthogonal feed axes, these machine tools are characterized by a substantially improved rigidity of the machine structure and by a comparatively high degree of local independence of their stability behavior. For carrying out, e.g., a straight movement by such machines, several or all movement devices must normally operate synchronously and at an interpolated speed. The degrees of freedom of movement of the movement devices necessitate a non-orthogonal coordinate system. Hence, a programmer is unable to realize the feed motions of the individual movement devices which are required for a movement of the support that is to be carried out relative to a stationary orthogonal coordinate system. Programming in the degrees of freedom of movement of the individual movement devices is extremely difficult.
In the case of such numerically controlled machine tools, it has therefore already been suggested that the position of a point of aim in coordinates of a fixed orthogonal coordinate system (X, Y, Z, A, B, C) should be transformed into a nonorthogonal coordinate system (L1, L2, L3, L4, L5, L6), while the program is being executed. German published patent application DE 195 22 963, for example, discloses a control for a hexapodal machine tool in the case of which a feed motion is subdivided into individual steps; calculating the starting and end points of each step, the necessary changes of length and rates of changes of the individual struts of a hexapodal suspension are determined on the basis of a starting point and a point of aim and on the basis of a predetermined step time. Detection of the actual condition is not carried out in this case.
In addition, it is known that such an interpolation cycle is followed by position control so that the control takes place in the coordinates of the strut lengths (L1, L2, L3, L4, L5, L6), since this would permit the operating values determined in the interpolation calculation to be transmitted to the struts in a comparatively simple manner.
Since the coordinate system of the strut lengths L1 is not orthogonal, each of the strut lengths L1 is normally a function of one or more coordinates of the orthogonal system (X, Y, Z, A, B, C). If the control takes place in the coordinates of the strut lengths, the individual control circuits will be coupled to one another. In view of non-linear interactions, the dynamic behavior of a machine tool controlled in this way is position- and direction-dependent. In particular, it is then not possible to adjust the dynamic properties with regard to the directions of the orthogonal system separately for each direction by means of suitable parametrization of the control circuits. Since the control paths associated with each individual controller in the coupled system have a higher order, the maximum adjustable loop gain is smaller than that of decoupled control circuits. If, however, the transformation of the coordinates took place before the interpolation, the effects of non-linear interpolation methods could no longer be mastered.
A control method and a control device of the type set forth at the outset are known from P. Rojek et al., xe2x80x9cSchnelle Koordinatentransformation und Fxc3xchrungsgrxc3x6ssenerzeugung fxc3xcr bahngefxc3xchrte Industrieroboterxe2x80x9d Robotersysteme, 2:2, pp. 73-81, Springer Verlag (1986). The control is here, however, performed in the non-orthogonal region. Decoupling of the control in the individual orthogonal coordinate axes is not possible. In addition, European published patent application EP 0 120 198 discloses a control device for an industrial robot in the case of which movement control is performed partly in orthogonal coordinates and partly in non-orthogonal coordinates.
It is an object of the present invention to provide a method and a device for controlling the movement of a support and of a machine tool, respectively, by means of which a dynamic behavior of a support with non-orthogonal feed axes is achieved which corresponds to that of a support with orthogonal axes of movement.
For solving the above-mentioned object, a method for controlling the movement of a support relative to a base is provided, wherein the target-condition values of the movement of the support are predetermined in coordinates of an orthogonal coordinate system of the base, the actual-condition values of the support are detected in the coordinates of a non-orthogonal coordinate system, which is defined by the degrees of freedom of the movement devices for moving the support, and transformed into the orthogonal coordinate system. A control for determining operating values for the movement of the support is performed on the basis of the target-condition and actual-condition values, said control being carried out in the orthogonal coordinates of the base, and the operating values are subsequently transformed into coordinates of the non-orthogonal coordinate system.
In addition, this object is achieved in the case of the above-mentioned device by the features that a computing unit is connected between the central control unit and the movement devices, said computing unit comprising a transformation module for transforming actual-condition values of the support from non-orthogonal coordinates into orthogonal coordinates, a controller for determining operating values in orthogonal coordinates from the actual-condition and target-condition values, and a further transformation module for transforming the determined operating values into the coordinates of the non-orthogonal coordinate system.
A separation of the control circuits of the orthogonal coordinate system is achieved in this way, so that the dynamic properties with regard to the axes of the orthogonal coordinate system can be adjusted independently of one another. This permits an optimization of the loop gain. In addition, the position- and direction-dependence of the dynamic properties is eliminated conceptually. The possibilities which exist in numeric control systems and which permit a compensation of the inertia in the individual controlled axes can only be utilized if the dynamic behavior of the axes is not position-dependent. This is achieved by the present invention. Hence, determined dynamic properties are created in the orthogonal axes, which now permit the use of compensation algorithms in devices with non-orthogonal axes of movement.
Preferably, a separate control unit is provided for each coordinate means of the orthogonal system so that the dynamic properties can be determined by parametrizing the individual control circuits separately from one another.
Preferably, a drive-bus controller is provided in addition to the device for generating the target-condition values, said drive-bus controller being used for controlling the individual movement devices. This drive-bus controller is connected to the central control unit via a computer bus. Due to the additional connection of the computing unit, which includes the controller and the transformation modules, it is possible that, on the basis of comparatively small modifications, an existing numeric control for a machine with orthogonal movement devices can be used for a machine with a non-orthogonal movement device so that the whole function of the control device can be continued to be used without any limitations. An operator who is familiar with known NC machine tools can thus use the machine with non-orthogonal movement devices in the manner which is already known to him. In addition, it is not necessary to adapt already existing NC programs so as to produce desired sequences of movements, such as a machining program for a machine tool or a program for positioning a workpiece.
When a digital drive bus to the movement devices is used, said drive bus is preferably implemented as a ring bus, a separate interface being provided on the computing unit for each arm of said ring bus. The bus is used for transferring the actual-condition values of the individual movement devices to the computing unit and for transferring the operating values to the movement devices.
The machine tool, which is suggested as well, achieves the same advantages as the above-mentioned device.
The device according to the present invention and the method according to the present invention can, fundamentally, be used in the case of devices having non-orthogonal axes of movement and a closed-loop kinematic structural design. Such use preferably takes place in the case of machines with a hexapodal suspension of the support on the base, the non-orthogonal coordinate system of the support being defined by the axial lengths of six struts of adjustable length. It is also possible to use, e.g., struts of fixed length whose foot points are moved in a translatory or rotatory manner.
For increasing the control accuracy in the case of digital control, also speed and force/moment values can be taken into account in addition to position values. Preferably, the movement devices are controlled via operating values corresponding to the moments of the individual movement devices required for the movement. This can be done, e.g., by adjusting the current of a motor. In principle, it is, however, also possible to use other movement devices, such as hydraulic or pneumatic cylinders. Also, the use of servomotors is possible. The operating values must then be adapted accordingly.
Other advantageous further developments of the present invention are described below and set forth in the dependent claims.